DISTRIBUTION IN TIME— THE GEOLOGICAL RECORD 457 



fied rocks, the pages of the geological record, are the deposits 

 formed in ancient seas and ancient lakes, or more rarely on old 

 land-surfaces, in the same way that sand, or mud, or ooze is now 

 accumulating on the floor of the existing ocean, in existing bodies 

 of fresh water, or, it may be, on land. Such old deposits, how- 

 ever, have usually undergone more or less consolidation, and 

 those which we now find above-water owe their present position 

 to movements of elevation, such as are even yet in progress in 

 certain parts of the world. But as these movements are generally 

 extremely slow, they usually produce no obvious result in the 

 brief span of a human lifetime. Remembering that a particular 

 stratum or layer of rock (and of necessity its fossils) is older than 

 those which rest upon it, and younger than those which underlie 

 it, geologists have been able to arrange 

 the different strata in their proper chrono- 

 logical sequence, and thus to construct a 

 continuous geological record, often pic- 

 turesquely known as the "record of the 

 rocks ". The fossils of the record obviously 

 afford some idea, though necessarily an 

 imperfect one, of the successive faunas of 



the globe for many millions of years, how many can only be 

 conjectured. A hundred millions is a common estimate, based 

 on many different kinds of evidence. 



Geological Periods. — Without entering into details which 

 may be found in any text-book of geology, it may be stated that 

 the geological record can be divided into four great epochs, which 

 are, beginning with the youngest: 



KAINOZOIC EPOCH (Gk. kainos, recent; zoe, life).— Age of Birds 

 and Mammals. 



MESOZOIC EPOCH (Gk. mesos, middle; zoe).—Kg& of Reptiles. 



PALEOZOIC EPOCH {Qx'k. palaios, ancient; zoe).—KgQ of Amphi- 

 bians, Fishes, and Invertebrates. 



EOZOIC EPOCH (Gk. eos, dawn; soe).—hg(t of Unknown Life. 



The time represented by these four epochs is of very unequal 

 length, but the Kainozoic, in which we live, has endured for a 

 much shorter period than the Mesozoic, which in its turn was 

 briefer than the Palseozoic, while possibly the Eozoic was longer 

 than the other three put together. The entire geological record 

 includes stratified rocks to a thickness of over 100,000 feet, a 



Fig 1312 — Strata in Vertical Section 



